The present invention relates generally to formation of finely divided powder at low cost.
More particularly, it relates to apparatus and methods for formation of particles having particle sizes generally in the range of 25A to 40.mu. or larger. The invention is useful in forming fine particles from high melting materials. By high melting materials is meant materials which melt at temperatures of about 600.degree. C. and above. However, the method also has unique application to materials of lower melting point. For example, it is very difficult to form fine particles of 25A to 40.mu. from some lower melting materials, such as cadmium, which melts at about 321.degree. C. However, the present invention can successfully accomplish this. It has great value for materials of higher melting point because of the great difficulty of reducing certain higher melting materials such as refractory metals and superalloys to finely divided form.
Very finely divided particles of high melting metals are needed for many applications. Until now, it has been difficult and costly to produce such very fine particles. One reason is that only a small percentage of very fine or ultrafine particles are formed in conventional processing of material into finely divided form. Conventional atomization by flowing gas may produce less than ten percent and in some cases less than two percent of particles in the range of less than 10 microns. When this is the case, fifty pounds of material must be atomized to produce one pound of ultrafine powder. The need to process large amounts of material to obtain the ultrafine fraction raises the cost of the ultrafine material which is collected and then separated from the coarser powder. Ultrafine powder is defined herein as powder having an average particle size of less than 10 microns, that is, a powder in which the more than half of the weight of the material is of a particle size of less than 10 microns.
Another problem in the production of ultrafine powder by conventional means, as by gas atomization, is that the finest particles tend to cling to larger particles. A particle of 50 Angstroms may cling tenaciously to a particle of 100 microns so that when the material is screened the ultrafine particle is collected with the larger particles rather than with the finer particles. This tendency of the finer to cling to larger particles further raises the cost of the ultrafine product.
For these and similar reasons the cost of ultrafine powder, depending on the nature of the material, may be hundreds or even thousands of dollars per pound.